The invention relates generally to the field of receiver selecting systems which select the most useable communications signal produced by one of a plurality of receivers. The invention more particularly relates to such systems in which each receiver output signal is transmitted over an ordinary telephone communication line to a central voting (selecting) control center.
In many radio communication systems designed to service a large area, it is often desirable to provide a number of different receivers, each positioned at a different location within the area to be serviced. The receivers will have overlapping effective reception ranges so that any message transmitted within the area will be picked up by at least one receiver. When a transmitted signal is received by several different receivers, a selection of the receiver producing the most desirable detected output signal should be made, so that only this signal is selected for use by the communications system. However in many such systems, one receiver will not continue to produce the best signal during the entire transmitted message because of changes in atmospheric conditions and changes in the transmitter location (e.g., when the transmitter is attached to a moving automobile). Thus the receiver selection decision must be either continuously or periodically updated. Several prior art systems do not update the receiver selection decision and thus are unable to always monitor the best received signal.
Most of the prior receiver voting systems couple the detected audio signal to a comparison location where the signal selection is then made. The prior systems do not compensate, or even attempt to compensate, for the variable system losses which can occur in the different signal paths between the receivers and the comparison location. Therefore prior systems can mistakenly select a receiver because the actual receiver producing the best detected audio signal is coupled to the comparison location by a signal path which has more or less loss associated with it than other signal paths. When the signal coupling apparatus consists of telephone transmission lines, as is the case in the present invention, the transmission losses can vary from hour to hour. Thus in a telephone line coupled system, the variable transmission losses may degrade the total communication system integrity.
Some prior systems produce a DC signal at the receiver, which is related to the quality of the detected audio signal, and transmit this DC signal to the comparison location. These systems cannot use standard telephone transmission lines. Other prior art systems produce coded audio tones at the receiver which indicate the quality of the detected audio signal. These coded tones are transmitted simultaneously with detected audio signal over standard telephone lines. At the signal comparison location, notch filters remove these signal quality coding tones from the received audio signal. The resultant frequency spectrum reduction of the received audio, caused by these filters, can severely impair the quality (fidelity) of the selected audio signal.
Another prior art system uses a signaling tone, which exists sequentially with the detected audio signals, to indicate whether the receiver is in a squelched or unsquelched condition. This prior system detects the signal quality of the audio signal received at the comparison location by using a log amplifier followed by an envelope detector followed by a valley detector to produce a DC signal indicative of the noise level of the incoming audio signal. The system uses a tone detector to determine when the receiver is squelched or unsquelched and thereby prevents the selection of a squelch indicating tone as the best (lowest noise level) incoming audio signal. The tone detector in this prior art system cannot be designed to rapidly respond to the squelch indicating tone without creating a audio signal falsing problem. This system also has no provision for equalizing the variable losses contributed by the transmission lines between the receivers and the comparison location. Also, the signal initially chosen in this prior art system is randomly selected and therefore will not normally correspond to the best audio signal produced by a receiver.
In some of the prior systems, no indication is given of which of the plurality of receivers is producing the selected signal. This information defines the optimum transmitter to receiver signal path. By not providing or using this information, these prior art systems are inefficient during any signal transmissions from the central control center location to the transmitter.